That equation is an idealization. If you study actual performance curves, compressibility and other factors neglected by that equation cause the wing to perform worse than that formula predicts. But it does give you a reasonable approximation at low mach numbers. It is most accurate at low altitude and low g-loadings where the aircraft speed tends to be lower, the speed of mach 1 is at a maximum, and the mach number of the air flowing over the wing is at a minimum and thereby minimizing the effects of compressibility.
It takes advanced computer modeling and a detailed 3-d plot of the aircraft (especially the wing and airfoil) to accurately predict aerodynamic performance at high speeds and high g-loads. In the end, even with the United States incredible lead in aerodynmic modeling software, only the actual test flights of the full size aircraft will provide data you can bank on.
I appreciate HTCs problem in modeling WWII aircraft. The majority of the aircraft for that timeframe do not have the kind of performance charts needed to nail the flight models. I assume that they use all reliable datapoints available and interpolate the remainder of the curves using the basic equations and good old human judgement. It seems that everytime someone provides better data from a relaible source, they re-evaluate the interpolation and judgement calls.
It would do the flight sim community a world of good if owners of warbirds would fully restore them to combat weights and performance, then do the necessary flight testing to produce the standard peformance charts used for modern jet fighters showing specific excess power, sustained g, instaneous g, cornering velocity, and roll rate as a function of speed and altitude. Of course, this would be an expensive process and would be of little benefit to anyone outside of those playing flight sim games. Personally, I think NASA could learn a lot by evaluating the differences between all these high performance aircraft, especially as they approach transonic speeds. It would probably help in their effort to make propellor airliners that are as fast as the turbofan ones of today.
